1280 
2. CaF transmitting the radiation along a triangular axis, gave 
a pattern identical with ZnS. 
3. Topaz, transmitting radiation in the direction of the bisectrix 
of the acute angle of the optical axes, gave a pattern which can 
be explained by assuming the net of the molecules to be built up 
from parallellograms with equal sides in the plane perpendicular to 
the bisectrix, and by points perpendicularly placed above the net 
points obtained in this way. 
From the photogram I calenlated the angle of the pg. It 
amounts to 66°10’. A trying of this angle with the angles of the 
planes of the prism, known from crystallographic data, gives a 
suitable agreement. [ hope to have an opportunity to calculate the. 
proportion of sides ete. for more types of Bravais nets. We may 
suppose that in this way we shall obtain the possibility of deciding 
between the different structure theories, and of coming to a rational 
description of crystals. 
4. The experiment of reflecting RÖNTGEN rays on the cleavage 
plane published by Brace in “Nature” of 23 of Dec., was repeated with 
mica. Because of the plate being longer exposed this time, there 
appeared on the plate, besides the reflected spot upon the planes 
parallel to the cleavage plane already found by Brage, also a number 
of other points of which by far the greater part were lying upon 
an ellipse rather changed into a circle. For plane of incidence the 
principal cross-section had been chosen, the photographic plate was 
placed perpendicular to the plane of incidence. The circle was lying 
asymmetrically, although the plane of incidence had been chosen 
in a principal cross-section. 
Supposing the monoclinic net for mica to exist in a rectangle (in 
the cleavage-plane) and a side inclining with respect to this rectangle, 
lying in a plane perpendicular to the cleavage plane, then in order 
to explain the patterns we must take for the proportion of the sides 
of the rectangle and the inclining side 8:13: 100, and besides we 
must suppose the angle of the cleavage plane and the inclining side 
to amount to 85°. The pattern obtained can still better be explained 
by using the second net of the monoclinic system. The basis then 
is a pg with very long and almost equal sides, and an angle of 
about 85° between the short diagonal and one of the sides. The 
third side is perpendicular to the pg considered, the rectangle through 
the short diagonal of the basis is centric. The cleavage plane then 
is // to this rectangle. This structure shows for mica an approach 
to the hexagonal type. 
The same results were shown by the pattern obtained when 
